Some of the piper voices are a step up from Apple’s. But I also have it automatically cycle through my preferred voices, which reduces monotony. piper is feee and on GitHub and doesn’t use much cpu. I had to comb through many voice to find the most realistic and natural sounding. (Some of the truly human sounding voices would hallucinate gibberish occasionally, so I abandoned those.)
I set this up because Apple’s reader would choke on text with special characters and abort reading, or read aloud bullet point markers strangely, or suddenly start reading all subsequent English text as if it was French after encountering a French name.
I love it so much, and in no small part to IINA respecting media keys and headset buttons, so I can be listening to an article and remove an AirPod and the article stops. This makes it so much easier to use around interruptions.
It's extremely good at mimicking voices, and it sounds quite natural even with those custom-generated voices.